1. Field of the Invention
This invention relates to a shift actuator providing fluid pressure to a piston slidably installed in a cylinder, whereby driving a shift operating member for gear shift by the piston.
2. Description of the Related Art
In the conventional art, there are transmissions having a synchronous mechanism for moving a synchro sleeve by a shift fork. And in these transmissions, as shown in Japanese Pat. No. 5280311 (patent literature 1), some have a shift actuator for driving a shift fork. The shift actuator includes a piston slidably installed in a cylinder, a fluid pressure chamber defined opposite to the piston in the cylinder to be supplied with fluid pressure such as oil pressure, and a shift fork (a shift operating member) driven by the piston, whereby the shift fork is driven to the shift direction (the axial direction of a fork shaft) by the piston which slides in the cylinder due to the fluid pressure supplied to the fluid pressure chamber.
Hereupon, in a shift actuator described in the patent literature 1, no mechanism for discharging hydraulic oil in a cylinder to the outside has especially been provided. Therefore, micro foreign matters (contamination) such as metal powder included in hydraulic oil have been likely to remain in the cylinder. This causes the micro foreign matters to be possibly bitten in a gap between an inner circumferential surface of the cylinder and an outer circumferential surface of a piston, whereby the inner circumferential surface has been likely to be damaged and worn.
Related to the point, in the Japanese patent No. 4946217 (patent literature 2), as a technique for reducing the risk that contamination is attached to a spool outer circumference of a spool valve, a configuration which a discharging oil passage opening with corresponding to a whole-circumferential groove formed on an outer surface of a valve spool on a sliding portion of the valve spool and a spool hole is disclosed.
However, when the configuration including a whole-circumferential groove formed on the discharging oil passage and the spool described in the patent literature 2 is applied to a shift actuator, providing the discharging oil passage can cause enlargement of an outside dimension of the shift actuator. Furthermore, forming the whole-circumferential groove on an outer circumference of the cylinder increases machining processes of the cylinder, whereby a manufacturing process of the shift actuator can be complicated.